Anolis oculatus

History and taxonomy

The Dominican anole is locally known as the zandoli, or tree lizard. The indigenous Caribs considered its presence in their home to be a sign of "good spirits".

American paleontologist and herpetologist Edward Drinker Cope first gave a cursory description of the species, which he named Anolis alliaceus, on the basis of 16 specimens owned by the British Museum that lacked locality data. He later separately described Xiphosurus oculatus in 1879, from 13 specimens in the U.S. National Museum (now the Smithsonian); the name oculatus (Latin, "eyed") refers to its distinctive lateral spots. The two taxa were finally described as one species, Anolis oculatus, in 1888, by the German-British zoologist Albert Günther.

Because of its morphological variation, however, confusion remained as to whether the Dominican anole comprised multiple species or only one. This was resolved by the work of American herpetologist James D. Lazell, Jr. In 1959, he collected over 500 specimens from thirty localities on Dominica, which he reached primarily on foot or on horseback due to the inadequate road system. From this, he concluded that it was a single species, and in 1962 provided the first coherent account of its variation, organized by different regional populations that he classified as four subspecies: A. o. oculatus, found along the southwestern coastal area, with a holotype from the capital of Roseau (most closely corresponding to the original type described by Cope); A. o. cabritensis, along the northwest coast, with a holotype from the Cabrits peninsula; A. o. montanus, in central, high elevation rain forest, with a holotype from Fresh Water Lake; and A. o. winstoni, along the northeast coast, with a holotype from the village of Woodford Hill. These first descriptions applied only to males, however, and omitted certain morphological features such as scale variation. Lazell returned to Dominica in 1966 to collect new specimens, and in 1972 supplemented his original descriptions, including color illustrations of sexual dimorphism among the types.

Lazell noted that the subspecies he described corresponded with the "strikingly different" ecological zones on Dominica, which result from the extreme elevations on the small island interacting with prevailing winds to produce varying rainfall and vegetation. Later morphological and molecular studies determined, however, that there was no interruption to gene flow between the different populations, but instead clinal variation adaptive to local ecological factors. Researchers have, therefore, determined that there is no basis for the use of subspecies nomenclature. The former subspecies are now described as ecotypes based on their geographic range: the South Caribbean ecotype corresponds to the former A. o. oculatus; North Caribbean ecotype to A. o. cabritensis; Montane ecotype to A. o. montanus; and Atlantic ecotype to A. o. winstoni.

Description

The Dominican anole is medium-sized for anoles, with a maximum snout-vent-length (SVL) in males of 61–98 mm (2.4–3.9 in) depending on the population (with larger sizes correlating with higher altitudes), and tails of at least equal length; females are smaller in all populations. Adult males have a tail crest and a prominent, extensible throat fan that is often a bright yellow or orange. The throat fan is only rudimentary in females, and less brightly colored. Some populations have bright blue eyes. Like other anoles, Dominican anoles are capable of slight color change.

Their ground color varies from pale tan or ash gray to deep brown or green. Markings also vary significantly. Adult males in all populations have some combination of white or light-colored spotting distributed over most of their bodies. This manifests in different populations as small, evenly distributed spots; scattered groups of white scales; or streaks that run together in a marbled appearance. Males in some populations additionally have larger lateral spots ringed by irregularly shaped black splotches. Markings on females and juveniles are significantly lighter or less distinct, and black spots are extremely uncommon. Females and juveniles may additionally have lateral streaks or middorsal stripes.

The cause of the variability of the Dominican anole has been the subject of intensive study. Its morphological traits vary independently, such that the presence of one trait does not predict the presence of another. Some traits vary altitudinally and others longitudinally, and correlate with ecological factors such as rainfall and vegetation type. Different traits coincide, however, in regional ecotypes. Populations in drier habitats tend to be paler in color with marbled or blotched markings, while those in wetter habitats are deeper green. The same patterns are also seen in the highly variable A. marmoratus on Guadeloupe, a neighboring island group that has a range of habitats comparable to Dominica.

Ecotype descriptions

The North Caribbean ecotype (formerly A. o. cabritensis) is found in the most arid part of Dominica, in low, scrub-like woodland on the northwest, Caribbean coast. It has the most complex markings of any population, with bold, light-colored spots that run together to form irregular stripes or marbling. Males commonly have a series of prominent, lateral black splotches. Its ground color is predominantly gray or pale yellow brown, sometimes with a darker, occasionally reddish head region.

The South Caribbean ecotype (A. o. oculatus) is found on the south and southwest coast, which does not differ significantly from the north. It is the smallest in size and the palest in color and markings. It has a light tan to yellow ground color, with varying white spots that are typically indistinct. Lateral dark splotches are inconspicuous or absent. This ecotype is threatened by an invasive species (see Conservation).

The Montane ecotype (A. o. montanus) is found in high elevation rain forest located in central Dominica. It has a deep green ground color, which matches the moss-covered tree trunks on which they are mostly found. They have small, bluish-white secondary spots, and occasional lateral, black-ringed splotches on males like those found in the North Caribbean ecotype. The largest sizes are attained among this population.

The Atlantic ecotype (A. o. winstoni) is found along most of the Atlantic (east) coast of Dominica, which is wetter than the west coast. It is intermediate in size, and has a typically orange to chocolate brown ground color, with small, scattered white spots, like the Montane type.

Distribution and habitat

The Dominican anole is restricted to the island of Dominica, one of the few islands in the Lesser Antilles to have retained its original reptile and amphibian fauna over the last 200 years. It is one of two lizard species endemic to Dominica, the other being the Dominican Ground Lizard. It is the only native anole species on Dominica. It is present in all habitats and areas of the island up to around 900 m elevation and is generally abundant, though it is tending towards extirpation from the southwestern coastal region due to an invasive anole species (see Conservation). The coastal woodlands of Dominica have been particularly noted as unusually favorable for reptiles, with a biomass among the highest recorded for terrestrial reptile populations; Dominican anoles have been estimated to occur in that environment at a mean density of 2148 per hectare.

Ecology

Dominican anoles are semi-arboreal, and mainly forage on the ground for food. Its diet mainly consists of insects, but varies by habitat and season based on the available food, and with the size of the individual. It may also eat fruit and even small vertebrates. In xeric woodland habitats on Dominica's Caribbean coast, it mainly feeds on tiny ants, termites, springtails, and barklice. Montane populations, which reach larger sizes, rely more on larger and more soft-bodied food such as Oligochaeta and Orthoptera, although smaller adults and juveniles in rain forest environments will also mainly feed on ants.

The main predators of Dominican anoles are the Antilles racer, and birds including the mangrove cuckoo, thrashers, and kingbirds, all of which are mainly found in coastal regions.

Behavior

Both males and females are territorial. Male territories are roughly twice the size of female territories, and males will typically mate with females with overlapping territory. Female territories may overlap in areas with high population densities. Research has shown gene flow within the species to be determined by male migration, which suggests that females do not have significant mate choice preferences. Migration probably occurs during the juvenile stage, as male and female adults are mostly sedentary.

During active periods, Dominican anoles typically perch on a tree or other vertical object, facing downwards to scan the ground for food or other lizards, though this perch makes them vulnerable to predators. From this perch, males will extend and retract their brightly colored throat fans, both to attract mates and to ward off competitors. Males will also bob their heads or perform what look like "push-ups" using their front legs. An invading male may cause a confrontation, during which they will circle at close quarters, inflating their bodies and gaping their mouths to appear larger and menacing. Physical contact during these confrontations is unusual and brief when it occurs, rarely resulting in physical injury. The invader will usually retreat after these confrontations even if it is larger than the defending male.

Activity patterns vary between populations, though the species as a whole is generally most active during the cooler hours of the day. In xeric woodland on Dominica's west coast, it is active throughout the day with peaks at dawn and dusk, while in rain forest populations, it is typically inactive during the middle of the day, remaining at high perches. At night, the Dominican anole climbs to the tips of branches and sleeps clinging to leaves, where heavier nocturnal predators cannot reach them.

Dominican anoles in some populations are very tolerant of humans, allowing them to approach closely before retreating. It will seek shelter under stones or other ground litter.

Life cycle and reproduction

Dominican anoles are relatively long-living and late maturing compared to mainland anole species. Males mature at a size of 35 mm SVL, and females mature at 40 mm SVL, which they are likely to attain at an age of two or three months old. They breed throughout the year, with a peak at the end of the dry season, particularly in areas such as the north Caribbean coast that have more seasonal rainfall variation. Females are oviparous, and lay multiple clutches of eggs each year, with a short period between clutches. Females in captivity have produced eggs every 14 days. Each clutch typically contains only one egg, though clutches of two can occur without apparent relationship to season, location, or size of the females. Egg production alternates ovaries and overlaps cycles, with one ovary ending its cycle after the opposite ovary has begun. Eggs are laid sheltered under ground litter such as rocks or leaves.

Evolutionary relationships

Anoles in the Caribbean have been extensively studied as "one of the best known cases of adaptive radiation." The Dominican anole is considered part of the bimaculatus series of Caribbean anoles, which are found on Dominica and islands to its north in the Lesser Antilles, and are more closely related to other Caribbean anoles than to South American anoles. Lazell considered the Dominican anole "the most bizarre member of the bimaculatus group, and one of the most peculiar members of its huge and diverse genus." Its karotype is unique among that group, as it possesses two pairs of acrocentric macrochromosomes. On this basis, Lazell hypothesized that it "long evolved in isolation, and lacks genuinely close relatives."

Within the bimaculatus series, it belongs to the Guadeloupean-Dominican clade, which includes A. marmoratus on the Guadeloupe Archipelago, A. lividus on Montserrat, A. nubilus on Redonda, and A. sabanus on Saba. A. marmoratus terraealtae, found only on the island of Les Saintes located in between Dominica and the main islands of Guadeloupe, may be more closely related to the Dominican anole than to other A. marmoratus subspecies. A. leachi, found on Antigua and Barbuda, is a likely sister taxon to the Guadeloupean-Dominican clade. The Dominican anole is estimated to have emerged from this group as a separate species no earlier than 10 million years ago.

Genetics

Levels of gene flow are relatively high in Dominican anole populations over large areas of Dominica, even between different ecotype populations and different members of mitochondrial DNA (mtDNA) lineages. This gene flow appears to be dominated by male migration, and occurs at such high levels as to likely prevent evolutionary divergence of different populations.

Morphological variation between different populations consists of clinal patterns that are incongruent with one another, correlating instead with environmental variables such as rainfall and vegetation type. These patterns of variation are also generally inconsistent with mtDNA lineages. This variation is primarily the result of strong pressures from natural selection caused by differences in habitat, with phenotypic plasticity a possible, though less important, contributing factor.

The populations on the west coast are an exception, and there is a large, apparent barrier to gene flow between the North Caribbean and South Caribbean populations. Though ecological conditions are apparently constant from north to south, there is a transition occurring over just a few kilometers on that coast from the North Caribbean (formerly A. o. cabritensis) ecotype to the South Caribbean (A. o. oculatus) ecotype, with a greater genetic difference between the two populations than would be predicted from their geographic distance. This separation is likely the result of a volcanic event within the last 50,000 years, as the transition zone is marked by relatively recent lava flows.

Conservation

The Dominican anole is threatened by an invasive competitor, the Puerto Rican crested anole, which established itself in Dominica between 1997 and 2002, and has begun to supplant it in the southwestern coastal area surrounding the capital, Roseau. Within that range, the Dominican anole has become absent or rare. Because that is almost the entire range of the South Caribbean ecotype, researchers have recommended a captive breeding program to preserve genetic diversity. The Dominican anole may eventually face extirpation from most of Dominica, except for perhaps isolated pockets in rain forest at elevations where the Puerto Rican crested anole tends to be restricted to open areas.